The present invention relates to a frequency detector for obtaining rotation speed information and phase information of a rotating device such as a motor.
Such a frequency detector in the prior art for obtaining rotation speed information and phase information of the rotating device is disclosed, for example, in Japanese Patent Publication No. 2960/66. This frequency detector contains a detecting coil radially disposed on a disc, and a mgnetic disc rotating with a rotating device and disposed in close and coaxial relationship with the disc carrying the detecting coil so that frequency information of one frequency and phase information corresponding to the rotation of the rotating device and can be obtained from electric currents flowing through the detecting coil in accordance with the rotation of the rotating device.
In order to control the phase of an apparatus such as a motor, of which responsive speed is relatively slow, speed control is usually made to increase an apparent responsive speed or to increase an apparant cut-off frequency thereof. There are various methods of obtaining speed information from the motor so as to make such a speed control. Among the various methods, a method using a frequency generator which utilizes a frequency detector as an armature thereof is most simple and accurate.
Recently, a video apparatus, an audio set or the like generally uses a direct current motor of an opposite flat plate type which inlcudes a rotor and a stator each in the form of a flat plate, between which a magnetic air gap in the form of a flat plate is formed. A frequency generator necessary for effecting speed control of such a motor of an opposite flat plate type uses, for example, a frequency detector shown in FIG. 1. The frequency detector includes a conductive pattern 1 formed on a printed circuit board, and it is disposed in the magnetic air gap of the motor in opposite relationship to the rotor in the form of the flat plate to serve as an armature of the frequency generator, so that a frequency signal proportional to rotation speed of the motor can be obtained from terminals 2 and 3 of the conductive pattern 1.
The frequency detector shown in FIG. 1 includes the conductive pattern 1 formed on the substantially round printed circuit board with a plurality of straight conductive pieces 4, 4', 4", . . . , outer periphery conductive pieces 5, 5', 5", . . . and inner periphery conductive pieces 6, 6', 6", . . . successively connected to one another in series. The 2n straight conductive pieces 4, 4', 4", . . . (where n is an integer, and in this case n=4) are radially disposed on the round printed circuit board at regular intervals. Each of the outer periphery conductive pieces 5, 5', 5", . . . in the form of arcs of a concentric circle connects between outer periphery ends of the two adjacent straight conductive pieces alternately. Each of the inner periphery conductive pieces 6, 6', 6", . . . in the form of arcs of another concentric circle connects between inner periphery ends of the two adjacent straight conductive pieces alternately. Such a conductive pattern is disposed in opposite relationship with permanent magnets 8 attached on a rotor in the form of a flat plate thereby to produce a frequency signal proportional to the number of revolution from the terminals 2 and 3 in response to the rotation of the rotor.
Suppose that the pole number of the motor for applying such a conductive pattern is l, the rotation speed is N (r.p.s.) and the number of the straight conductive pieces 4, 4', 4", . . . is m, the frequency f [Hz] of the signal obtained between terminals 2 and 3 is given by the equation (2) under the condition of the following equation (1). EQU m/l=(2n+1) (1)
where n=0, 1, 2, . . . ##EQU1## In the example shown in FIG. 1, the pole number l of the motor is 8 and the number of the straight conductive pieces is 8. Therefore, m/l=1. This satisfies the condition of the equation (1). The signal frequency f can be obtained from the equation (2). Referring to FIG. 1, it will be noted that the direction of the electromotive force in the respective straight conductive pieces 4, 4', 4", . . . at a certain moment can be shown by arrows.
When the speed control for the motor or the like will be made on the basis of the output signal from the frequency detector as shown in FIG. 1, it is necessary to convert the frequency signal obtained from the frequency detector as a control signal for controlling the speed of the motor into a voltage signal. As a circuit for converting the frequency signal into the voltage signal, there are known a method of using a sample-and-hold circuit and a pulse counting method of effecting a counting operation in response to a rising edge of the frequency signal. In the method of using the sample-and-hold circuit, phase rotation appears due to a time required to sample the frequency signal and convert it into the voltage signal. In the pulse counting method, phase rotation also appears due to a time required to count the frequency of the frequency signal and to convert the count value to the voltage signal by a smoothing circuit. If the phase rotation between the frequency signal and the converted voltage signal is large, it is difficult to stably control the speed of the motor in accordance with the voltage signal since the frequency indicated by the voltage signal and an actual frequency at present are largely different. Therefore, in order to stably control the speed, it is necessary to minimize the phase rotation when the frequency signal is converted to the voltage signal. In either of the above two methods, since the phase rotation can be more reduced as a frequency of the signal indicating the speed information becomes higher, it is preferable to make higher the output frequency of the frequency detector for taking out the speed information for the same rotation speed of the motor.
On the other hand, in view of phase control of the motor, the gain of a phase detecting circuit in the phase control system becomes advantageously higher at a higher output frequency of the frequency detector acting as a phase information of the motor depending on the frequency of the reference phase signal used in the phase control system. However, if the gain is too high, it is difficult to stably control the phase. If operation of the phase control becomes unstable due to a too high gain of the phase detecting circuit, there is a method of inserting an attenuator in the output of the phase detecting circuit to reduce the gain. However, in this method, a range for effecting a phase lock of the phase detecting circuit becomes undesirably narrower. Thus, the output frequency of the frequency detector as the phase information has a proper value based on the decided system construction.
Therefore, it is necessary for phase control of an apparatus with slow responsive speed such as a motor to take out two signals with different frequencies for speed information and phase information. Consequently, in a conventional apparatus, two separate frequency detectors, for example, have been used to obtain two signals with different frequencies, or a single frequency detector for generating a high frequency signal for speed information is used to divide its output frequency and obtain a low frequency signal for phase information. However, there was a drawback of increasing a cost of the frequency generator remarkably in either of the two methods.